Use dedicated I/O executor

app/src/main/java/eu/faircode/email/DB.java

@@ -405,7 +405,7 @@ public abstract class DB extends RoomDatabase {
                 .databaseBuilder(context, DB.class, DB_NAME)
                 //.openHelperFactory(new RequerySQLiteOpenHelperFactory())
                 //.setQueryExecutor()
-                .setTransactionExecutor(Helper.getParallelExecutor())
+                .setTransactionExecutor(Helper.getBackgroundExecutor(4, "db"))
                 .setJournalMode(wal ? JournalMode.WRITE_AHEAD_LOGGING : JournalMode.TRUNCATE) // using the latest sqlite
                 .addCallback(new Callback() {
                     @Override
@@ -2665,7 +2665,7 @@ public abstract class DB extends RoomDatabase {
     public static void checkpoint(Context context) {
         // https://www.sqlite.org/pragma.html#pragma_wal_checkpoint
         DB db = getInstance(context);
-        Helper.getParallelExecutor().execute(new Runnable() {
+        db.getQueryExecutor().execute(new Runnable() {
             @Override
             public void run() {
                 try {
@@ -2695,7 +2695,7 @@ public abstract class DB extends RoomDatabase {
 
     public static void shrinkMemory(Context context) {
         DB db = getInstance(context);
-        Helper.getParallelExecutor().execute(new Runnable() {
+        db.getQueryExecutor().execute(new Runnable() {
             @Override
             public void run() {
                 try {

app/src/main/java/eu/faircode/email/Helper.java

@@ -280,7 +280,7 @@ public class Helper {
         }
     }
 
-    private static ExecutorService getBackgroundExecutor(int threads, final String name) {
+    static ExecutorService getBackgroundExecutor(int threads, final String name) {
         ThreadFactory factory = new ThreadFactory() {
             private final AtomicInteger threadId = new AtomicInteger();
 

app/src/main/java/eu/faircode/email/ViewModelMessages.java

@@ -200,7 +200,7 @@ public class ViewModelMessages extends ViewModel {
                     break;
             }
 
-            builder.setFetchExecutor(Helper.getParallelExecutor());
+            builder.setFetchExecutor(db.getQueryExecutor());
 
             model = new Model(args, builder.build(), boundary);
             models.put(viewType, model);